In the field of molecular nanoelectronics, semiconductor nanocrystals, nanowires and nanotubes are showing increasing promise as components of various electronic devices. Semiconductor nanocrystals, for examples, have physical properties significantly different from those of bulk materials. The strong dependency of electronic structure of semiconductor nanocrystals on the nanocrystal size and shape provides additional options for the design and optimization of their material properties [Murray et al., Annu. Rev. Mater. Sci., 30, 542 (2000)]. Moreover, the ability of semiconductor nanocrystals to form stable colloidal solutions allows their integration into electronic devices by inexpensive and high-throughput solution based processes like spin-coating and jet-printing. The films of close-packed nanocrystals exhibit extremely poor conductivities [Morgan et al., Phys. Rev. B. 66, 075339 (2002)], thus hindering their application in electronic devices. Recently it has been shown that electrochemical doping of semiconductor nanocrystals results in significant improvement of their conductivity [Yu et al., Science 300, 1277 (2003); Yu et al., Phys. Rev. Lett. 92, 216802 (2004)]. However, electrochemical doping is not suitable for use in solid state electronic devices because it requires the presence of liquid electrolytes. Thus, there is a need for alternative methods of doping nanocrystals.
Other nano-components such as semiconductor nanowires [Lieber et al., US Published Application US 2002/0130311 A1] and carbon nanotubes are also important elements of nanoelectronics. Nanotubes are unique for their size, shape, and physical properties, and depending on their electrical characteristics, have been used in electronic devices such as diodes and transistors.
Although much progress has been made on carbon nanotube (CN) based transistors in terms of both fabrication and understanding of their performance limits [Javet et al., Nature 424, 654 (2003); Javey et al., “Advancements in Complementary Carbon Nanotube Field-Effect Transistors”, IEDM Conference 2003; Wind et al., Appl. Phys. Lett. 80, 3817 (2002); Favey et al., Nano Lett. 4, 447 (2004)], there are still key issues to be addressed for potential technological applications. In particular, there has been no process-compatible doping method for CN field effect transistors (CNFET). Unlike doping in CMOS processes, CNFET cannot be doped substitutionally via ion implantation due to damages to the CN lattice. It is known that CNFETs fabricated from as-grown CNs under ambient conditions show p channel conduction due to oxygen interactions at the metal-CN interface [Derycke et al., Appl. Phys. Lett. 80, 2773 (2002)]. However, the oxygen content at the metal-CN interface can easily be changed by standard fabrication processes (e.g., any post processing involving vacuum pumping such as thin film deposition). In fact, a p-CNFET can be easily converted to an ambipolar or n-CNFET via vacuum pumping. Although n-CNFETs can be formed by alkali metals [Derycke et al., Appl. Phys. Lett. 80, 2773 (2002)] or gas-phase (NH3) doping [Kong et al., Science 287, 622 (2000)], a controlled environment is required to prevent dopant desorption, because upon exposure to air, these devices quickly degrade and may become non-operational. Shim et al. has demonstrated the use of polyethyleneimine (PEI) for n-doping of CNFETs [Shim et al., J. Am. Chem. Soc. 123, 11512 (2001)]. However, additional alternative methods are still needed to provide consistent and stable doping for technologically viable CNFETs.
For semiconductor nanowires, n-doping of nanowires using gas-phase dopants has been demonstrated [Greytak et al., Appl. Phys. Lett. 84, 4176 (2004)]. However, after a nanowire has been integrated into a device, it is not easy, or possible, to vary doping level along the nanowire using gas phase doping because the higher temperatures typically used in gas phase doping may not be compatible with other materials already present in the device. Alternative approaches such as solution phase processing provide various advantages, one of which is the ability to allow controlled doping along the nanowire at temperatures compatible with device doping.